We have developed a model system that is useful for the analysis of many aspects of epithelium formation. Epithelia form the bulk of tissues in the body and consist of sheets of apposed cells resting on a basement membrane. The first epithelium to form during mouse development is that which segregates as the trophectoderm from the inner cell mass of blastocysts; the second is the appearance of primitive endoderm on the surface of the inner cell mass. Our model system is an in vitro counterpart of this event. It utilizes F9 teratocarcinoma cells which differentiate to "embryoid bodies" in suspension cultures containing 5 x 10 to the -8 M retinoic acid. Embryoid bodies consist of an outer epithelium of visceral endoderm cells (characterized by alphafeto-protein production) with less differentiated inner cells and a central cyst. Epithelium formation occurs over 8 days, and our aim is to identify the key events which are necessary to initiate the process. We have established that the synthesis, secretion and/or accumulation of the basement membrane glycoprotein, laminin, is likely to be important and we would now like to examine the details of the changes that occur in matrix components during epithelium formation and in two other related processes. One is the loss of epithelial integrity (obtained by addition of dibutyryl cyclic AMP); the other is reformation of an epithelium after embryoid bodies are disaggregated and reaggregated. A second possible crucial process in epithelium formation is also to be studied; that is, the role of cellular interactions in the form of junctional complexes and desmosomes. We hope to correlate observations using biosynthetic measurements, immunological techniques and electron microscopy with the appearance (or lack) of the epithelium in our various culture systems. We will determine if the observations that we have made using teratocarcinoma cells are also true for the formation of visceral endoderm cells on inner cell masses isolated from mouse blastocysts to establish the universality or otherwise of our model system. The analyses described in this study have relevance to our knowledge of the development, maintenance and possible modes of dissolution of epithelia and therefore will increase our understanding of normal and invasive behavior of cells.